MPI_ALLTOALLV_INIT(3)				     Open MPI				     MPI_ALLTOALLV_INIT(3)

       MPI_Alltoallv  <#mpi-alltoallv>,	 MPI_Ialltoallv <#mpi-ialltoallv>, MPI_Alltoallv_init - All processes send
       different amount of data to, and receive different amount of data from, all processes

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Alltoallv(const void *sendbuf, const int sendcounts[],
	       const int sdispls[], MPI_Datatype sendtype,
	       void *recvbuf, const int recvcounts[],
	       const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm)

	  int MPI_Ialltoallv(const void *sendbuf, const int sendcounts[],
	       const int sdispls[], MPI_Datatype sendtype,
	       void *recvbuf, const int recvcounts[],
	       const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
	       MPI_Request *request)

	  int MPI_Alltoallv_init(const void *sendbuf, const int sendcounts[],
	       const int sdispls[], MPI_Datatype sendtype,
	       void *recvbuf, const int recvcounts[],
	       const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
	       MPI_Info info, MPI_Request *request)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
	       RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
	       INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
	       INTEGER COMM, IERROR

	  MPI_IALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
	       RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, REQUEST, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
	       INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
	       INTEGER COMM, REQUEST, IERROR

	  MPI_ALLTOALLV_INIT(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
	       RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, INFO, REQUEST, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
	       INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
	       INTEGER COMM, INFO, REQUEST, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Alltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
		       rdispls, recvtype, comm, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
	       TYPE(*), DIMENSION(..) :: recvbuf
	       INTEGER, INTENT(IN) :: sendcounts(*), sdispls(*), recvcounts(*),
	       rdispls(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Ialltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
		       rdispls, recvtype, comm, request, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
	       recvcounts(*), rdispls(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Alltoallv_init(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
			       rdispls, recvtype, comm, info, request, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
	       recvcounts(*), rdispls(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Info), INTENT(IN) :: info
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • sendbuf: Starting address of send buffer.

       • sendcounts: Integer array, where entry i specifies the number of elements to send to rank i.

       • sdispls: Integer array, where entry i specifies the  displacement  (offset  from  sendbuf,  in	 units	of
	 sendtype) from which to send data to rank i.

       • sendtype: Datatype of send buffer elements.

       • recvcounts: Integer array, where entry j specifies the number of elements to receive from rank j.

       • rdispls:  Integer  array,  where  entry  j  specifies	the displacement (offset from recvbuf, in units of
	 recvtype) to which data from rank j should be written.

       • recvtype: Datatype of receive buffer elements.

       • comm: Communicator over which data is to be exchanged.

       • info: Info (handle, persistent only)

OUTPUT PARAMETERS
       • recvbuf: Address of receive buffer.

       • request: Request (handle, non-blocking only).

       • ierror: Fortran only: Error status.

DESCRIPTION
       MPI_Alltoallv <#mpi-alltoallv> is a generalized collective operation in which all processes  send  data	to
       and  receive data from all other processes. It adds flexibility to MPI_Alltoall <#mpi-alltoall> by allowing
       the user to specify data to send and receive vector-style (via a	 displacement  and  element  count).   The
       operation  of this routine can be thought of as follows, where each process performs 2n (n being the number
       of processes in communicator comm) independent point-to-point communications (including communication  with
       itself).

	  MPI_Comm_size(comm, &n);
	  for (i = 0, i < n; i++)
	      MPI_Send(sendbuf + sdispls[i] * extent(sendtype),
		  sendcounts[i], sendtype, i, ..., comm);
	  for (i = 0, i < n; i++)
	      MPI_Recv(recvbuf + rdispls[i] * extent(recvtype),
		  recvcounts[i], recvtype, i, ..., comm);

       Process	j  sends the k-th block of its local sendbuf to process k, which places the data in the j-th block
       of its local recvbuf.

       When a pair of processes exchanges data, each may pass different element count and  datatype  arguments	so
       long as the sender specifies the same amount of data to send (in bytes) as the receiver expects to receive.

       Note that process i may send a different amount of data to process j than it receives from process j. Also,
       a process may send entirely different amounts of data to different processes in the communicator.

       WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

       When  the  communicator	is  an	inter-communicator, the gather operation occurs in two phases. The data is
       gathered from all the members of the first group and received by all the members of the second group.  Then
       the data is gathered from all the members of the second group and received by all the members of the first.
       The operation exhibits a symmetric, full-duplex behavior.

       The  first  group  defines the root process. The root process uses MPI_ROOT as the value of root. All other
       processes in the first group use MPI_PROC_NULL as the value of root. All processes in the second group  use
       the rank of the root process in the first group as the value of root.

       When  the  communicator	is an intra-communicator, these groups are the same, and the operation occurs in a
       single phase.

USE OF IN-PLACE OPTION
       When the communicator is an intracommunicator, you can perform an all-to-all operation in-place (the output
       buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of sendbuf. In  this  case,
       sendcounts,  sdispls, and sendtype are ignored. The input data of each process is assumed to be in the area
       where that process would receive its own contribution to the receive buffer.

NOTES
       The specification of counts and displacements should not cause any location to be written more than once.

       All arguments on all processes are significant. The comm argument, in particular, must  describe	 the  same
       communicator on all processes.

       The  offsets  of	 sdispls and rdispls are measured in units of sendtype and recvtype, respectively. Compare
       this to MPI_Alltoallw <#mpi-alltoallw>, where these offsets are measured in bytes.

ERRORS
       Almost all MPI routines return an error value; C routines as the return result of the function and  Fortran
       routines in the last argument.

       Before  the error value is returned, the current MPI error handler associated with the communication object
       (e.g., communicator, window, file) is called.  If no communication object is associated with the MPI  call,
       then  the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When
       MPI_COMM_SELF is not initialized (i.e.,	before	MPI_Init  <#mpi-init>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#mpi-finalize>,  or  when	using the Sessions Model exclusively) the error raises the
       initial error handler. The initial error handler can  be	 changed  by  calling  MPI_Comm_set_errhandler	<#
       mpi-comm-set-errhandler>	 on  MPI_COMM_SELF  when  using the World model, or the mpi_initial_errhandler CLI
       argument	 to  mpiexec  or  info	key   to   MPI_Comm_spawn   <#mpi-comm-spawn>/MPI_Comm_spawn_multiple	<#
       mpi-comm-spawn-multiple>.   If  no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
       error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error  handler  is  called  for  all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT  An  error handler that can be invoked on a communicator, window, file, or session. When
	 called on a communicator, it acts as if MPI_Abort <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#mpi-abort> was called on a communicator containing the  group
	 of processes in the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       See also:

	  • MPI_Alltoall <#mpi-alltoall>

	  • MPI_Alltoallw <#mpi-alltoallw>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				     MPI_ALLTOALLV_INIT(3)
